The tomato seed, 3-5 mm in size, has a silky appearance and contains a large coiled embryo surrounded by a small amount of endosperm; it nevertheless retains its viability for many years after harvesting. Well over 90% germination is possible after 10 years when the seed is stored under cool dry conditions. The first sign of germination is the appearance of the small white root (radicle) (Fig. 3). As the radicle pushes downward into the growing substrate, the hypocotyl (primitive stem part) takes on a crook-like form known as the plumular hook. The plumular hook grows to the soil surface, where in response to light it begins to straighten and turns green. When the seed is firmly anchored in the soil and the plumular hook is straight, the colyledons (seed leaves) are pulled out of the seed coat (testa), which remains in the soil. However, when the growing medium is too loose the colyledons cannot separate from the testa, and sometimes the seedling is distorted (Fig. 3). Tomatoes have a well-defined taproot, with an abundance of lateral fibrous roots. It is possible to encourage the development of more fibrous roots by pruning the taproot, as happens when a seedling is pricked out from a seeding tray and transplanted into a pot. The plant readily forms adventitious (aerial) roots on the stem, which is of great value if the roots become diseased or damaged; a layer of moist soil or peat (the latter is preferable) at the stem base encourages new roots to form at this point. Once the cotyledons are fully grown the true leaves soon appear at the growing point.
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